There has been proposed an X-ray diagnostic apparatus having a system (MAF (Micro Angiography Fluoroscope System)) in which, in front of a conventional flat panel detector (to be referred to as detector A hereinafter), another detector (to be referred to as detector B hereinafter) can be arranged. Detector A in the MAF system has a larger detection element size than detector B and/or a standard spatial resolution.
Detector B in the MAF system is arranged in front of detector A. Detector B in the MAF system has a smaller detection element size than detector A and/or a high spatial resolution. Detector B is attached to a C-arm through a holder mechanism. The holder mechanism supports detector B so as to make it movable between a park position and an X-ray irradiation range.
When acquiring a series of images while rotating the C-arm, it is necessary to correct the vibration of the C-arm and the incompleteness of the rotational orbit of the C-arm, in order to obtain an accurate three-dimensional reconstructed image. These corrections are achieved by the geometrical calibration of a detector. In a conventional angiography system, a calibration table used before reconstruction is generated from a series of images obtained by imaging a specific calibration phantom.
In the X-ray diagnostic apparatus having the MAF system, a holder supports detector B. In rotational imaging using detector B, therefore, sagging and vibration occur in detector B with respect to the original position of detector B. In a related art, the correction of the geometrical position of detector B has the following two problems.
First, the above calibration table is generated based on the assumption that the incompleteness of the rotational orbit of the C-arm and the vibration of the C-arm repeatedly occur each time. However, in an X-ray diagnostic apparatus having an MAF system, the features of the vibration of the C-arm (to be referred to as vibration characteristics hereinafter) change because of a change in the distribution of masses between detector B at the park position and detector B arranged in front of detector A. A change in the vibration characteristics of the C-arm breaks the assumption that the vibration characteristics of the C-arm are invariable when the vibration of the C-arm repeats and the C-arm rotates at two different positions relative to detector B. This poses a problem that the above calibration table is inappropriate for the correction of a geometrical position relative to detector B.
Second, since the visual field size of detector B is smaller than that of detector A, it is not appropriate to use, for detector B, the calibration phantom for the calibration of a geometrical position relative to detector A. In general, when using a calibration phantom, a detector having an appropriate size is required to generate image data with high accuracy and to cover the field of view.
However, detector B typically has a smaller field of view than detector A. For this reason, requiring a different calibration phantom for detector B will increase the manufacturing cost of an angiography system and increase the complexity of a calibration procedure. The use for detector B of a calibration table designed for detector A does not produce a satisfactory result in reconstruction using the projection data obtained by rotational angiography because of additional vibration of the holder caused by potential mechanical instability of the holder mechanism with respect to detector B.
That is, when reconstructing volume data based on an output from detector B in an X-ray diagnostic apparatus having an MAF system, the related art requires a phantom suitable for the visual field size of detector B to correct the geometrical deformation and vibration of the C-arm including a holder mechanism. In this case, since accuracy is required when manufacturing a phantom dedicated to detector B, the cost will increase. In addition, it takes much time to perform calibration for detector B.
Furthermore, if the angle of a C-arm with respect to the vertical axis is large (e.g., 90°), the sagging amount of detector B increases due to the influence of gravity. For example, as shown in FIG. 13, if the position of a collimator blade when the angle of the C-arm is 90° coincides with the position (FIG. 14) of the collimator blade when the angle of the C-arm is 0°, since the position of detector B moves in the vertical direction, the irradiation range of X-rays becomes inappropriate.
FIG. 13 shows a state in which detector B which is moved in front of detector A is arranged at a side surface (at a position of 90°) of an object (top plate). In this case, no X-rays are detected at the upper end portion of the detection surface of detector B because of the sagging of detector B. In addition, the sagging of detector B makes the collimator blade overlap the lower end portion of the detection surface of detector B, resulting in shielding X-rays. FIG. 14 shows a state in which detector B which is moved in front of detector A is arranged at the front surface (e.g., a position of 0°) of the object (top plate). In this case, the collimator properly limits the X-ray irradiation range.
As shown in FIG. 13, since those of the collimated X-rays which do not reach the X-ray detection surface of detector B are not visualized, the object is unnecessarily exposed to X-rays. In addition, in this case, since part of the collimator blade covers part of the X-ray irradiation range which corresponds to part of the detection surface of detector B, the detection surface of detector B cannot be effectively used. Furthermore, when using detector B in a very narrow visual field, the visual field desired by the operator may be blocked by the collimator blade.
Conventionally, a virtual projection image (e.g., a blood vessel image, an image similar to an X-ray image, or a three-dimensional road map image) is sometimes generated based on the geometrical information of a C-arm and an X-ray optical system (a tube focus and an FPD) and volume data acquired in advance (the three-dimensional image obtained by the C-arm, CT (Computed Tomography) volume data, MRI (Magnetic Resonance Imaging) volume data, or the like). The generated projection image is superimposed/displayed on an actually acquired X-ray image.
When acquiring an X-ray image, if the angle of the C-arm relative to the vertical axis is large (e.g., 90°), the sagging amount of detector B increases due to the influence of gravity. On the other hand, the object is placed on the top plate, and hence its position is invariable. For this reason, for example, as shown in FIG. 14, the X-ray irradiation range differs from the ideal irradiation range because of the sagging of detector B. On the other hand, the projection image generated based on the volume data acquired in advance is generated based on the ideal geometrical information of the C-arm and the X-ray optical system. For this reason, as shown in FIG. 15, this image shifts from the actually acquired X-ray image.
“A” in FIG. 15 indicates an ideal case without any sagging of detector B. In this case, when detector B which is moved in front of detector A is arranged at a side surface (a position of 90°) of the object, the X-ray irradiation range associated with the image obtained by detector B coincides with the virtual X-ray irradiation range associated with the projection image of volume data. That is, in an ideal case without any sagging of detector B, the positional relationship with the image obtained by detector B coincides with that of the projection image of the volume data.
“B” in FIG. 15 indicates a case with the sagging of detector B. In this case, when detector B which is moved in front of detector A is arranged at a side surface (a position of 90°) of the object, the X-ray irradiation range associated with the image obtained by detector B differs from the virtual X-ray irradiation range associated with the projection image of the volume data. That is, the positional relationship with the image obtained by detector B differs from that with the projection image of the volume data because of the sagging of detector B.